Leads associated with an implantable medical device (IMD), such as a cardiac pacemaker or an implantable cardioverter-defibrillator, typically include a lead body containing one or more elongated electrical conductors. The electrical conductors extend through the lead body from a connector assembly provided at a first lead end proximal a housing of an associated implantable medical device to one or more electrodes located at the distal lead end or elsewhere along the length of the lead body. The conductors connect stimulation and/or sensing circuitry within the implantable medical device housing to respective electrodes or sensors. Each electrical conductor is typically electrically isolated from other electrical conductors and is encased within an outer sheath that electrically insulates the lead conductors from body tissue and fluids.
Cardiac lead bodies are continuously flexed by the beating of the heart. Stress can also be applied to a lead body by patient movement, during implantation, during lead repositioning, or during IMD changeout. Such stresses may lead to fracture of one or more conductors of the lead. Additionally, the electrical connection between the implantable medical device and the lead can be intermittently or continuously disrupted, which may result in intermittent or continuous changes in lead impedance.
Short circuits, open circuits or significant changes in impedance may be referred to herein as lead related conditions. Sensing of an intrinsic heart rhythm through a lead can be altered by lead related conditions, and structural modifications to leads, conductors or electrodes may alter sensing integrity. Furthermore, impedance changes in the stimulation path due to lead related conditions may affect sensing and stimulation integrity for pacing, cardioversion, or defibrillation.